Process of refining alkaline pulps



Patented Aug. 19, 1930 UNITED STATES PATENT OFFICE onoaen A.- RICHTER, or BER IN, NEW HAMPSHIRE, Assionoa 'ro nnowu coxrAHx,

- or BERLIN, EW HAMPSHIRE, A CORPORATION or MAINE ?ROCESS OF REFINING ALKALINE PULPS m5 hrawing.

This invention has relation to the production of a white high, alpha cellulose fiber, i. e.. a fiber containing a high percentage of alpha or resistant cellulose, possessing high 5 strength and tearqualities, from a pulp resulting from the digestion of Wood chips in an alkaline liquor, and more particularly-in a kraft or sulphate liquor;

Pulp produced by an alkaline process of n digestion, such as kraft, is especially characterized by its brownish color, doubtless arising from the considerable amount of ligneous and other coloringsubstance and less-resistant celluloses contained therein, and also by its quality of high strength, which-renders such pulp ver valuable commercially for use in the pro notion of strongpaper's, viz., wrappingand bag papers, where the characte'ristlc of color is relatively unimportant.

Such pulp, however, is almost exclusively restricted to the manufacture of wrap ing or bag papers, and ordinarily cannot e employed for purposes where a pulp of high purity or high alpha cellulose content is required; nor is it ordinarily used in the manuacture of writing papers where a white pulp is desired. 7

Moreover, such pulp is exceedingly diflicult to bleach. While it is possible to bleach such pulp withan excessive amount of bleach, say, about of 35% bleach (CaOCI based on the weight of dry pulp, the strength of the resultant product is ow and its alpha cellulose content thereby suffers a material decrease by conversion or degradation into oxycelluloses .so that the commercial value of the product is lowered. This difiiculty in bleaching may be directly traced to the high percentage of ligneous and other coloring substance remaining in the kraft' pulp after the alkaline digestion.

In the course of research work, I have discovered, generallystated, that if an alkaline cooked pulp, such as kraft, is treated with an oxidizing or bleaching agent in the presence of a large excess of a strongly alkaline liquor such as a solution oi caustic soda, the pulp is purified or freed from less-resistant celluloses, ligneous and other coloring impurities,

'nying black spent digesting liquor.

without perceptible injury to the alpha cel-' Application filed November 25, 1925. Serial No. 71,437.

lulose content thereof, and is thereby converted into a refined and easily bleachable pulp which is'high in alpha or resistant-cellulose and, because of its characteristics is adapted for a great variety of uses which will subsequently be pointed out.

The kraft pulp employed inthe present invention may be produced as ordinarily by the digestion of wood chips in a digester under the requisite time, temperature, and pressure conditions in accordance with modern practice, in the usual kraft or sulphate liquor, containing sodium sulphide, sodium hydroxide,-and but an adventitious or relatively slight amount of sodium sulphate. Spruce, hemlock, pine, straw, or any other .raw cellulosic material suitable for the production of kraft pulp may be utilized. After the necessary 'period of digestion, the digester contents are blown and the kraft pulp is washed and separated from its accompa- The washed pulp is then preferably screened to remove shives, specks and other contaminating matter and is then thickened to the consistency desired for admixture with the alkaline liquor containing the oxidizing agent.

The thickened pulp is then intimately mixed with a concentrated solution of caustic soda containing a relatively small proportion. of bleach'or oxidizing agent, and is treated therewith, say at room temperature (30 C.) for a sufiicient period of time to permit the bleach and alkali to react with and remove the less-resistant celluloses, ligneous and other coloring impurities contained in the pulp. In carrying out the treatment or puri fication, too high a concentration of caustic soda must be avoided, since otherwise the fiber will become mercerized, such mercerization tending to lower the strength of the pulp, and in such case subsequent beating merely macerating or shredding the fiber without efi'ecting its hydration so that paper made therefrom is extremely weak. On the other hand, if too low a concentration of caustic soda is employed, suflicient reaction of the impurities contained in the pulp with the caustic soda solution will not .take place and the resulting pulp will not be sufiiciently high in alpha cellulose. In other words, the

-concentrat1on of caustic soda employed in the at which a suflicient purification of the pulp .is not effected. These limits will vary somewhat, depending upon the temperature of treatment employed and differences in characteristics of the initial raw stock. After the purification, the pulp, containing about 93% alpha cellulose, is washed free from its treating liquor and may be bleached to a high degree of whiteness with a moderate amount of hypochlorite bleach without materially injuring its strength or decreasing its alpha cellulose content. If a ulp of maximum whiteness is desired, the leachedpulp may be sulperbleached in a chlorine solution.

W ile I have indicated that the rocess is ordinarily carried out at a relatively low temperature, e. g., room temperature, it may be stated that lower than room temperature (say, C. to "O.)maly be emplo ed to effect the purification of t epulp. Li ewise higher temperature, for example, C. to 75 C., and a correspondingly suitable concentration of caustic soda, may be employed in such treatment. For optimum results in respect to. alpha cellulose content in a finished product prepared from 'a representative kraft stock at a treating temperature of about 30 (3., the minimum and maximum (NaOOl) is used, the bleach may concentrations in a stock-sus 'nsion of, about 5% to 6%. are, respective y, about 100% and175% caustic. soda based on the wei ht of dry fiber. The concentration of beach may vary from 5% to 20% of 35% bleach (either NaOCI or CaOCl based on the dry weight of pulp. If desired, where a soluble bleach such as sodium h pochlorite introducing chlorine (C1 into the treating liquor until the deslred concentration of bleach is formed therein.

While I am unable to explain recisel what occurs in 'the purification oft e kra pulp, it may be that: the oxidation of the diflicultly removable li neous or coloring subsoda solution late the'degree o the reaction of the oxidizing agent to permit only a mlective re'adtion between it and the less resistant or easily reactable substances, but also reacts with such substances and dissolves out from the pulp products of both reactions. in any event, whatever the action be underpul --gan 1 c impurities removed from the pulp,'but.

made by 'l quired beforesuch treatment to its a1 or by first treating the pulp with an alkaline solution and then subjectin the treated pulp to an oxidizing reaction. hat is to say, the

results obtained by practicin the rocess of the present invention are the treatment of the kraft pul with the bleaching or oxidizing agent w ile in the presence of a large excess of alkaline agent, to supplement as well as dizing reaction. v

The alkaline oxidizing treatment is carried out, preferably whlle stirring or agitating the pulp so as to promote a uniform epen ent upon to regulate the oxireactlon sa at room temperature, for about five to six ours, depending upon the characteristics of the particular stock undergoing treatment. The resultant pulp, if the treatment is carried'out under the optimum conditions stated, will contain from 93% to 97% alpha cellulose. The pulp may beseparated and washed substantially free from alkaline and entrained products of the reaction, by passage. through a counter-current washer, and the spent liquorth'us separated from the will not only contain the various ora ver high .percenta sume caustic soda. spent liquor is preferably 'e-employed in subsequent'treatment'of ot er kraftipul and the chemicals in the winder of we spent liquor'may be recovered.

The washed pulp is of light color, not much different in appearance from the color of ordinar unbleached sulphite ulp, and, as state contains 93% to 97% a pha cellulose. Since the treatment has removed from the of residual 'uncon helarger portion of loo original'kraft pulp a substantial amount of igneous and other colorin substance, together with less-resistant ce uloses, the removal of such bleach-consumin constituents allows the pulp to be bleache" with made" rially less hypochlorite or bleach than is reroduce 'a j purewhite product, and thus the leaching" operationmay be effected without'injury to the fiber strength ofthepulp or reduction of Ordinarily a w a: "cellulose ."content.v I bleaching treatment at ajstock consistency of about 10% to 16% with"about5% to 10% of 35% lime bleach (CaOClg), based on the dr I weight of pulp, is sullicient to bring the p11 pup to awhiteness equivalent to or 1111-.

proximating thatv of ordinary bleached sulphite pulp. v a

. The stock thus bleached may be improved in color by a superbleaching'operation, which maybe carried-out at a, stock consistency of from 4% to 6%; with about .2%-to 15% chlo rine based on-the Wei ht of dr fiber.

lying the purification of the pulp,- it'may Due to its high a pha ee ulose contenta' rived from an alkaline digestion of wood chips, which process comprisestreating such pulp at low temperature with a liquor containing bleach and a preponderant amount of caustic-soda, said caustic soda being present in amount sufficient to react with non-alpha cellulose constituents contained in such pulp but insufiicient to efi'ect a substantial mercenzation of the alpha-cellulose content thereof,

suificient to; react with non-alpha cellulose constituents contained in such pulp but insufiicient to effect a substantial mercerization of the alpha cellulose contentthereof, wash in" the treated pulp, bleaching the washed pu p at a stock consistency of about 10% to 16% with about to of lime I and superbleaching said bleache stock. consistency of from 4% to 6% with.

bleach based on the dry wei ht of pulp, and. superbleachin said bleach 2% to 5 chlorine. based on the dry weight 0 P p consistency of a lit 5% to 6% and at about pound 'for a In the production'of a. white fiber characteriz'ed by its high alpha or'resistant cellulose content, a processwhich comprises treatmg alkaline diggegted wood pulp at astock" step which comprises treating an alkaline digested wood pulp in a liquor containing an oxidant and a large excess of alkaline com-V riod of time suflicient to efiect 'a removal 0 alarge proportion of. the nonalpha cellulose constituents of such ulp.

12. In the ized'b its hi alpha cellulose content, that production of fiber c aracter- I g step W ich comprises treating an alkaline digested. wood pulp at about room temperature and under atmospheric pressure in a liquor containing an oxidant and a large exce'ss of alkaline compound for a period of time suflicient to effect a removal of alarge proportion of the non-alpha cellulose constit-uents of such pulp.

In testimony whereof I have aflixed my signature. I

' GEORGE A. RICHTER.

pulp at a stock consistency o from 4% to 6% with about room temperature for about five or six hours,

. with a li uor containing 5% to 20% of 35% 4 bleachaOCI and about to 175% caustic soda, based. on the weight of dry pul washing the treated pulp, bleachingthe was edpulp at a stock consistency of about 10% to 16% withabout 5% to 19% of 35% limebleach based on the dry weight of pu Pu P t a about 2% to ,5% chlorinelbased onthedry weightofpulp. g 9.5111 .theproduction ,offlber characterized byits high alpha or 'resistantcellulose con- .te'iit,' a step which comprises treating alka line digested wood pulpina li uor-eontain ing an oxidizing agent and cau's ic soda-said f 22in! containing about 5%to 10% caustic a a a 10. A process which comprisestreati wood pulp in a liquor containing both a preponderatmg proportion of caustic soda and a smaller but substantial-proportion of an oxidant until the non-alpha cellulose content of such pulp has been largely dissolved,

and bleaching the treated pul' j 11. In the reduction of her characterby its big pha llulose content, that.

'1 lie .1 l V Refined stock (before bleaching) Kraft stock 93-97 Tan or yellowish.

From the foregoing table, which re resents average figures obtained in testing iiierent batches of pulp, it is to be observed that the tear test of the initial raw pulp has undergone a decided increase, while the strength of such pulp has been preserved substantially in entirety. Moreover, it is to be noted that theinitial raw pulp which was relatively low in less-resistant beta and gamma cellulose but high in pentosans (which characteristics are doubtless to be ascribed to the alkaline digestion of the raw cellulosic material in producing such ulp) has been convertedinto a refined stock, notably high in alpha cellulose content, due to a material in its ntosan content.

Be ore concluding, it may be stated that, while I have confined myself to a description of the purification of kraft pulp, I have also discovered that a long fibered soda pulp approximating the strength of kraft, i. e., a pulp produced from the digestion of raw cellulosic material, such as spruce and pine, may be treated by the process herein described to produce a hi h al ha cellulose substantially like that produced from a kraft stock. The various conditions under which the treatment is carried out may vary somewhat, depending upon the characteristics ofthe soda pulp undergoing treatment.

I do not herein claim the subject matter of application Serial No. 716,153, filed May27, 1924, b Milton 0. Schur and myself wherein a wood maintained alkaline during the leration. 'In the process of that ap lication the alkaline content of the liquor wiiile sulficient to maintain an alkaline condition of the mass during the entire bleaching operation, is quite su ordinate to its bleach content, whereas in the process of the present applireduction "cation-the alkaline content of the' treating liquor is considerabl in excess of its bleac content and the woo pulp undergoing treatpulp is bleached in a li uor which is.

leaching op-- ment is one derived, more particularly an alkaline digestion of wood chips.

Having thus described this invention, it should be obvious that various changes might be resorted to in the rocedure under which the process is carried out, but without departin fromthe spirit or scope of the appende claims. I

What I claim is:

1. In the production of fiber characterized by its high alpha or resistant cellulose content, a step which comprises treating chemical wood pulp in a liquor containing both an oxidizing afgent I and an excess of alkaline compound or a period of time 'suflicient to eifect a removal of a large proportion of the non-alpha cellulose constituents contained in such pulp.

2. In the production of fiber characterized by its high alpha or resistant cellulose content, a step which comprises treating at a relatively low temperature with a liquor containing bleach and a large excess of caustic soda, pulp derived from an alkaline digestion of wood chips, said caustic soda preponderating in amount and bein present in amount suflicient to react with a arge proportion of the non-alpha cellulose constituents contained in such pulp but insuiiicient to efiect a substantial mercerization of the alpha cellulose content thereof.

3. In the production of a white fiber characterized by its high alpha or resistant cellulose content, a step of treating pulp derived from an alkaline digestion of wood chips, which process comprises treating such pulp at a stock consistency of about-5%, to 6% and at about room temperature for about five or six hours, with a li uor containing 5% to 20%. of 35% bleach aOCl and about 100% to 175% caustic soda, based on the weight of dry pulp.

4. In the production of a fiber characterized by its high alpha or resistant cellulose content, a process which comprises'treating alkaline digested wood pulp in a liquor containin' bleach and a large excess of caustic soda, or a period of time sufiicientto effect the removal of a large proportion of the nonfrom a period of time suflicient to I 6. .In the production of a white fiber characterized by its high alpha or resistant cellulose content, a process of treating pulp delac 

